Concerns over the environmental consequences of burning fossil fuels and their resource constraints, along with the growing world demands in energy, have led to increasing penetration of renewable energy generated from sources such as solar and wind. However, the intermittent and varied nature of the renewable resources can make integration and dispatch of the renewable power challenging. Electrical energy storage (EES) can provide a solution to integration and dispatch of renewable power. Among the most promising EES technologies are redox flow batteries. A redox flow battery is an electrochemical device that is capable of storing up to megawatt-hours (MWhs) of electrical energy via a reversible electrochemical energy conversion.
In a redox flow battery, electrical energy is converted instantly to chemical potential (charge) or vice versa (discharge) at the electrodes as the negative and positive electrolyte solutions flow through the cell, and the electrical energy is stored as reduced and oxidized ionic species in the electrolyte solutions.
A redox flow battery has characteristics that render the battery suitable as an energy storage system: a redox flow battery can store and release electricity on demand; a redox flow battery can tolerate fluctuating power supplies and repetitive charge/discharge cycles at maximum rates; redox flow battery cycling can be initiated at any concentration of redox-active elements in the electrolytes; the power and energy of a redox flow battery can be separately designed, which can offer great flexibility to stationary applications; a redox flow battery potentially delivers a long cycle life; a redox flow battery can be safe, because the flowing electrolytes can carry away the heat generated from the electrode reactions and ohmic resistances; a redox flow battery can be charged at a rate that is as fast as or much faster than that of discharge; and a redox flow battery can store MWhs of electrical energy and can offer multi-MW power in a simple design and system setup.
The performance of a redox flow battery can be susceptible to decrease over time and/or usage. The concentration of each redox-active ion in the cathode electrolyte and the anode electrolyte and the respective volumes of the cathode electrolyte and anode electrolyte can provide an indication of battery performance, which can be monitored over time or at any state of battery life, for a given operating condition. The concentration of each redox-active ion in the cathode electrolyte and the anode electrolyte and the volumes of the cathode electrolyte and anode electrolyte can provide an indication of the energy in a redox flow battery that is available to meet application demands (e.g., peak shaving, wind power storage, voltage stabilization, etc.) presented by the connected load or end user. Therefore, there is a need to determine the concentrations of redox-active ions at any time during a redox flow battery's life cycle to determine side-reactions, make chemical adjustments, periodically monitor battery capacity, adjust performance, or to otherwise determine a baseline concentration of the redox-active ions for any purpose.